Ion beam etching has been used in various applications, for example in etching for manufacturing magnetic read sensors, magnetic writers, sliders and the like for disk drive data storages, as well as in sputtering targets for film formation.
Homogeneous broad ion beams are needed to obtain good etching uniformity. For example, the homogeneous broad ion beam can be achieved by controlling a plasma density profile in the vicinity of porous electrodes (a grid) for ion extraction, or by zoning the grid. In this respect, the plasma density profile means a change in the charge density of plasma observed in the traverse direction of a plasma chamber (for example, an ion beam source) in the vicinity of the bottom wall of the plasma chamber. In the case of a cylindrical plasma chamber, the plasma density profile is measured along the diameter of the cylindrical plasma chamber near and above the bottom wall. The zoning means adjusting the diameters of individual holes (also referred to as apertures) in the grid in order to compensate for the non-uniformity of the plasma. Although this zoning is effective, the effect is limited to certain grids.
It is desirable to provide the plasma density profile with the uniformity equivalent to the required etching uniformity. An electromagnet coaxial with the ion beam source chamber may be used for improving or controlling the plasma density in the plasma chamber. Generally speaking, a magnetic field generated in the center of an electromagnet is formed in the axial direction of a typical cylindrical plasma chamber. The electromagnet may be placed near an upper or side wall of the plasma chamber (see Patent Document 1).
Another method of controlling the plasma density profile is to use a movable plug or a movable recessed container provided on the upper wall. Generally speaking, the plasma density is highest in the middle of the chamber; and as the plug is placed to extend inside the plasma chamber from the vicinity of the center of the upper wall, the plasma portion is changed in shape, whereby the plasma density profile becomes more homogeneous. The movement of the plug is useful to control the plasma density profile in various process conditions. An electromagnet or permanent magnet may be additionally placed inside the plug for the purpose of controlling the plasma density profile under the plug, or confining plasma around the plug to the vicinity of the edge of the bottom surface of the plug. The above-mentioned plasma shaping method is effective to compensate for tolerances of component parts, and to compensate for slight variations in the grids which occur after long-term use or regeneration processing for cleaning. For a workplace that requires multiple tools and processes for compensation, the capability of controlling the plasma density profile is very helpful, for example, in obtaining a specific etching profile for correcting the non-uniformity on the workpiece resulting from a preceding process.
Meanwhile, as disclosed in Patent Document 2, plasma in an ion beam chamber can be changed in shape by use of a movable plug for the purpose of controlling the plasma density profile.
FIG. 1 is a cross-sectional diagram of a conventional plasma ion beam source disclosed in Patent Document 2. In FIG. 1, a plug 2 is inserted in the inside of a cylindrical plasma chamber 1 from an opening formed in the upper wall of the plasma chamber 1. A grid assembly 4 is provided in the bottom portion of the plasma chamber 1. An RF coil 5 is provided around the side wall of the plasma chamber 1. A gas introduction port 6 is provided in the upper wall of the plasma chamber 1. Furthermore, an O-ring 3 is provided between the wall surface of the opening formed in the upper wall of the plasma chamber 1 and the plug 2 inserted in the opening. The plug 2 is configured to be movable in arrow directions in FIG. 2. Moreover, a bottom surface 2a of the plug 2 is provided with an extended portion 7 in a predetermined shape for fine adjustment of the distribution of the plasma.
In FIG. 1, an inert gas (argon, xenon, krypton or the like) is introduced into the plasma chamber 1 from the gas introduction port 6. Once high-frequency power is applied to the RF coil 5, plasma is generated inside the plasma chamber 1. Ions are extracted from the plasma by the grid assembly 4 as extraction electrodes to each of which a predetermined voltage is applied, and forms an ion beam. The ion beam is radiated to a member (substrate) to be processed.
The grid assembly 4 includes a first electrode (screen grid) 4a, a second electrode (acceleration grid) 4b and a third electrode (deceleration grid) 4c arranged in that order from the inner side of the plasma chamber 1. Each of the first electrode 4a, the second electrode 4b and the third electrode 4c is a porous plate electrode having a grid structure with multiple holes. From a viewpoint of life extension and durability, molybdenum or carbon having a low sputtering rate is used as their component material (grid material). As shown in FIG. 2, the first electrode 4a is connected to a first power supply (not illustrated), and maintained at the positive potential; the second electrode 4b is connected to a second power supply (not illustrated), and maintained at the negative potential; and the third electrode 4c is connected to the earth.
In this configuration, when the plasma is generated in the plasma chamber 1 and the positive voltage and the negative voltage are respectively applied to the first electrode 4a and the second electrode 4b, only ions in the plasma in the plasma chamber 1 are extracted by electrostatic acceleration by the grid assembly 4 due to the difference in potential between the first electrode 4a and the second electrode 4b. Thereby, an ion beam 24 as shown in FIG. 2 as emitted from the ion beam source. In this respect, the angle of deflection of the ion beam 24 extracted from third electrode 4c with respect to the central axis of the hole is referred as to a beam divergence angle θ. The technique disclosed in Patent Document 2 adjusts the plasma density profile by moving the plug 2 in the arrow directions in FIG. 1.